We propose to establish DNA clones whose RNA transcripts are found exclusively in the brain cytoplasm (brain-specific) and use them to strengthen our overall research project on the gene expression in the nervous system. The overall objective of this research is to understand why brain transcripts have an unusually large complexity compared with those of other tissues. For this, we study the transcriptional complexity (the fraction of rat genome expressed as RNA) of various rat tissues, tumors, and their derivative cell lines, with special emphasis on the nervous system. In order to make our research more incisive, we propose to isolate DNA clones which represent a portion of the genome specifically transcribed into messenger RNAs (mRNA) of the brain. DNA fragments of the entire rat genome have been amplified in a bacteriophage lambda, from which we will select those for brain mRNAs. We expect to isolate several classes of brain-specific DNA clones with different quantitative levels of transcription, i.e., rare, abundant, and intermediately abundant classes. These clones will be extremely useful in furthering our studies of gene expression in the brain and other organs.